Fundamental Concepts to Achieving Superior Repeatability

February 13, 2020

The following concepts were originally presented by Jim McKenzie of Hendrick Engines Dynamometer Labs during the 1998 AETC conference. These are intended as guidelines for using an engine dyno and obtaining maximum repeatability in the results. Following these guidelines can result in +/- .1% repeatability. SuperFlow engineers added additional guidelines to get the most from your SuperFlow product.

The Dyno Cell

  • Airflow across the engine is a must. It must be sufficient to evacuate the test cell at least 8 to 10 times per minute. Look for 1-1.75” water pressure drop in your test cell.
  • Engine intake air should be sealed off from the environment inside the test cell. The engine should draw its air from the shop if possible or use outside shop air. Additionally, you may add a fan to purge air in the plenum.
  • Use some type of exhaust system and keep it sealed as well as possible. Many times you will work in the test cell with the engine running. Hazardous fumes emit unless you use some type of exhaust. Also, consider noise abatement requirements for your area and use mufflers where required. Critical-grade industrial mufflers are a good choice. You want little or no back pressure in your exhaust system.
  • Make sure your water system is adequate for your testing needs. You will use approximately 10 gallons per minute for every 100 horsepower (5-6 gpm if you are not running a cooling tower). To rapidly warm up engines, a separate preheated water system can be employed, but SuperFlow recommends use the dyno water system for engine cooling. Pressurized cooling towers increase repeatability and will allow you to test the engine under more realistic conditions.

The Dynamometer

  • Calibrate, calibrate, calibrate. Ensuring your dyno’s torque measurement system is correctly calibrated is critical to obtaining repeatable results. The Hendricks team calibrates before and after each test. Other teams calibrate every morning and keep a log of calibration results. All use precision weights. Also Ensure correct calibration values are loaded for your fuel and air turbines.
  • Keep your dyno in good operating condition and follow the proper maintenance procedures. Change the absorber oil at least every 60-100 hours of operation with 10w30 synthetic oil (60cc for standard absorber, 120cc for 871 absorber). Keep your dyno and test cell clean. Preciseness comes from cleanliness.
  • Keep spare parts available. SuperFlow recommends a zero-downtime kit consisting of servo valves, engine-speed pickups, water filters, and absorber pump seals. These parts and others are essential to have on hand during crunch time.
  • Oil and water temperature management is absolutely essential for repeatability. SuperFlow sells a pressurized cooling tower and an oil cooling heat exchanger. Invest in both for optimal performance from the engine.
  • Fuel-system management temperature is critical. Fuel flow and pressure rates should be consistent. Fuel temperature must be maintained to get repeatable Brake-Specific Fuel Consumption (BSFC) and Air/Fuel (A/F) ratio numbers. Fuel temperature is important for power repeatability.
  • Airflow measurement is crucial. It provides Brake-Specific Air Consumption (BSAC) and volumetric efficiency numbers. Make sure your air turbines are calibrated. Send them back to SuperFlow yearly to have them checked. Keep your calibration numbers available so you can verify the proper ones are loaded in your software.
  • If your dyno system does not have a temperature-compensated load cell, get one from SuperFlow. These eliminate the need to constantly adjust the torque zero function on the dyno throughout a day’s test session and then readjust the next morning when the load cell is cold. Failure to do this affects repeatability.
  • Buy a mercury barometer to obtain the correct barometric pressure reading for calibrating the dyno. Correct the reading for temperature and gravity.
  • You must input correct vapor pressure often and accurately. By adding the automatic temperature/humidity probe to the system, accuracy is improved and becomes automatic. You are no longer relying on a psychometric chart and your ability to read wet and dry bulb temperatures accurately.
  • Put WinDyn software on your system. It provides much finer analysis of dyno data through its plotting capabilities. The DOS software and terminal-based systems cannot compare.

The Operator

  • Be smooth and consistent.
  • Start every test record with the same oil and water temperatures.
  • Develop appropriate test methodologies and routines specific for your application. For instance, Hendrick Engines tests from 5400-8700 on open motors and 4900-7300 on restrictor plate motors. They use 100-rpm acceleration rates and critically monitor oil and water temps. Allow the engine to stabilize at an rpm before pressing Start to begin the test.
  • Average your test data before attempting to analyze it. The WinDyn software allows you to average multiple tests together. Use a minimum of three tests. Engines do not repeat well, so by averaging the tests, you get a much more realistic picture of what the engine is doing. You can also average columns of data in WinDyn.
  • Always optimize the engine after any changes. Try to return to the same A/F ratio setting before determining if the change made an improvement or not.
  • Optimize the A/F ratio for atmospheric conditions. Hendrick Engines uses O2 sensors in each cylinder and one in each collector. They believe O2 sensors are crucial for repeatability.
  • Try to minimize the correction factor, if possible. Don’t dyno on bad air days.
  • Systematically return to the baseline configuration during the course of your testing. This ensures the improvements you saw were from the changes you made.
  • As atmospheric conditions change, re-establish the correct A/F ratio.

The Engine

  • The engine must be properly broken in using your established break-in procedure.
  • The engine’s state of tune is critical for good repeatability. A badly running engine will not repeat.

Tuning Tips

  • Remember, G-forces cannot be duplicated on the dyno, so jetting on the dyno will be different than jetting required on the track.
  • Jet for maximum power, and let BSFC fall were it may.
  • Use step testing to jet and to work on BSFC. Step testing is far more accurate than acceleration tests for jetting work.
  • Typically, for every 100-rpm change in acceleration rate, you lose approximately 2 to 3 lb-ft of torque. Most circle track teams test at 100-rpm acceleration rates. Drag racers use 300- or 600-rpm rates.
  • The best way to measure A/F ratio is through the use of O2 sensors. Hendrick Engines uses 10: one in each header tube and one in each collector.
  • The air turbine must be on straight and sealed to get repeatable results.

For more helpful information, you can view tips on improving repeatability over at DYNOmite Dynamometer.